A freshly cut meat surface isn’t bright red; it’s naturally purplish because it contains the purplish-red muscle protein, myoglobin.
But when myoglobin is exposed to oxygen in the air, it quickly turns into bright, cherry-red oxymyoglobin. That’s why only the outer surface of your ground beef is that nice, bright red color that we generally associate with freshness; the inner parts haven’t been exposed to enough air.
Freshly cut, purplish beef is shipped from the packing house to the markets in airtight containers. After being ground at the market, it is usually wrapped in a plastic film that permits the passage of oxygen, and the surface of the meat then “blooms” with the red color of oxymyoglobin.
But on longer exposure to oxygen, the red oxymyoglobin gradually oxidizes to brownish metamyoglobin, which not only looks bad but gives the meat an “off”-flavor. It’s this metamyoglobin-brown color that signals over-the-hill meat. In reality, however, this transformation happens long before the meat becomes actually unwholesome.
Retail markets use plastic packaging materials (either low-density polyethylene or polyvinyl chloride) that allow just enough oxygen to penetrate to keep the surface of the meat at the bright red oxymyoglobin stage.
To sum it up: If your beef, whether cut or ground, is a dull purple, it’s really very fresh. But even if it has gone brown with metamyoglobin, it can still be good for several days.
Your nose, not your eyes, is ultimately your best sense organ for determining whether your hamburger is too brown.